Interactions between tranexamic acid and protein were studied in respect of the antifibrinolytic actions of tranexamic acid. Tranexamic acid did neither show any interaction with fibrinogen or fibrin, nor was incorporated into cross-linked fibrin structure by the action of factor XIII. On the other hand, tranexamic acid bound to human plasmin with a dissociation constant of 3.5 X 10-5 M, which was very close to the inhibition constatn (3.6 X 10-5 M1 for this compound in inhibiting plasmin-induced fibrinolysis. The binding site of tranexamic acid on plasmin was not the catalytic site of plasmin, because TLCK-blocked plasmin also showed a similar affinity to tranexamic acid (the dissociation constant, 2.9-4.8 x 10-5m). in the binding studies with the highly purified plasminogen and TLCK-plasmin preparations which were obtained by affinity chromatography on lysine-substituted Sepharose, the molar binding ratio was shown to be 1.5-1.6 moles tranexamic acid per one mole protein. On the basis of these and other findings, a model for the inhibitory mechanism of tranexamic acid is presented.